1. Field of the Invention
This invention pertains to a relay switch, and more particularly to a mercury displacement relay switch wherein the displacement plunger is supported by a spring submerged in the mercury body and which is uniquely secured within the relay.
2. Description of the Prior Art
The normally-closed mercury displacement relays of the prior art generally comprise a cylindrical electrode containing a body of mercury, a pin electrode sealed within the cylindrical electrode and insulated therefrom, the pin electrode further having a portion thereof in contact with the mercury, a displacement plunger within the cylindrical electrode and partially submerged within the mercury, and a coil means adapted to the cylindrical electrode for moving the displacement plunger from its first position to a second position above the first position upon being energized. The electrical connection is opened when the coil means is energized drawing the displacement plunger from its first position upwardly to its second position, whereby the mercury level recedes and breaks contact with the pin electrode.
The displacement plunger is generally positioned in its first position by either a spring or a weighted ballast. In the prior art relays which use a spring to position the displacement plunger in its first position, the spring is placed between and connected to the top of the displacement plunger and the bottom of the cover of the cylindrical electrode, and is maintained out of contact with the mercury body. The primary disadvantage of this structure is the presence of the spring within the arcing environment of the relay. Upon the making or breaking of electrical contact between the mercury body and the pin electrode, intense arcing occurs in the air space above the mercury body, which causes structural deformation and the eventual structural failure of the spring. This presents the undesirable circumstance of having to replace the whole mercury displacement relay due to the failure of the simple spring device therein.
In the normally-closed prior art mercury displacement relays utilizing the weighted ballast to maintain the displacement plunger in the first position, a problem different from that above occurs, which also makes this type relay undesirable to use. Since mercury has a specific gravity of 13.6, the weighted ballast must have a numerically high density in order to overcome the buoyant nature of the displacement plunger within the mercury body. The most commonly used weighted ballast having the required density characteristics is tungsten or a tungsten alloy, which is placed upon the top of the displacement plunger within the cylindrical electrode. Although the tungsten ballast is not undesirably affected by the arcing within the cylindrical electrode, its use as a weighted ballast in the manufacturing of a large number of relays becomes prohibitively costly.
Both relay devices above have their own distinct undesirable features and, consequently, the use of one in place of the other does not remove all of the problems described above.